Lots of APIs pop out JSON.
JSON schema let's you make that a bit safer.
My point is that if they need this "safer", why not pop out XML instead.
The largest benefit of JSON is that is not XML. That it doesn't get bogged down in schema's, strong typing, interfaces, contracts and so on. You loose all that by turning JSON into XML-written-in-a-different-syntax.
I fail to see any benefit of a rigid, typed, schema'd JSON that XML does not offer more naturally.